The commercial usefulness of many technologies is limited by the materials they utilize. Many intermetallic materials have yet to achieve their commercial promise due to challenges in the preparation of high quality crystalline product. These challenges often stem from physical characteristics of the compound's constituent elements. Since improvements upon existing materials are often not enough to provide significant advancements, new materials, structures, and/or approaches must be found. As an example, intermetallic clathrates are a novel class of materials currently being widely investigated for the potential they hold to impact and address a number of materials limited problems in energy conversion applications. The realization of the expected properties for these materials may revolutionize the commercial viability of important applications such as thermoelectric power generation and photovoltaic energy conversion. However, one of the substantial challenges to this realization lies in the preparation and crystal growth of high quality materials, which includes precise control over chemical composition, production of high quality crystalline product, and an inexpensive, reproducible, and safe method for manufacturing these materials. The present invention addresses these problems, and offers a new, industrially accessible and cost-effective method for crystal growth of intermetallic materials which cannot be prepared by conventional techniques, as well as a number of other materials as well. A key advantage of the invention is the ability to grow high quality single crystals of materials including constituent elements with greatly differing melting points and vapor pressures, or both.
Several applications/markets will benefit from the new crystal growth method presented herein, such as: high efficiency photovoltaic solar cells; high efficiency thermoelectric modules for power conversion, for use in automotive waste heat recovery, onboard power generation for deep space probes (NASA) and military applications; light detectors that may be integrated into current Si-based technologies; various optoelectronic devices; novel semiconductor materials for electronics applications; electronic devices with low thermal conductance; semiconductor devices operating at elevated temperatures (i.e. >300 K); thermoelectric cooling devices used for infrared sensors, fiber optic guidance systems, charged coupled devices, thermal reference sources, laser diodes, pump lasers, refrigerators/freezers, bio-analytical instrumentation, DNA amplification, high performance liquid chromatography, and supercritical fluid chromotography; and thermal management for biological systems.
As stated above, one of the significant challenges to high quality crystal growth of many intermetallic clathrate compositions, in particular compositions that exhibit technological and commercial usefulness such as the clathrate-II NaxSi136 (0<x<24), are the vast difference between the melting points of the constituent elements, the vast difference in vapor pressures of the constituent elements, and the need for control of the partial pressures of the constituents for control of reaction products. These challenges exclude the use of conventional crystal growth techniques for preparation of these materials. Although the NaxSi136 clathrates have been known for more than forty years, crystal growth of these materials has remained elusive.
The novel invention disclosed herein is a method that is a marked improvement over previous methods of making NaxSi136 clathrates. Prior art in the field has traditionally centered on the production of microcrystalline powders by thermal decomposition. (see U.S. Pat. Nos. 6,461,581 and 6,797,199 to Eguchi et al.) For example, in U.S. Pat. No. 6,423,286 to Gryko, a method for making NaxSi136 clathrates was described, however the products of this method still contained measurable amounts of impurity phases. Moreover, the products were microcrystalline powders, not single phase pure crystals. The present invention, in contrast, produces Na24Si136 as high quality single crystals, and also in completely phase pure form, by a method that could readily be utilized in industry.
U.S. Pat. No. 6,103,403 to Grigorian discloses a method of manufacturing a silicon clathrate film that is bonded to a silicon substrate. The method produces a silicon clathrate layer and an intermediate n-type silicon layer on the silicon substrate. This patent does not produce phase pure high quality crystals as are produced in the present invention.
U.S. Pat. No. 7,534,414 to Nolas et al. discloses a method for the production of type II clathrates in thin and thick film form for use in photovoltaic (PV), thermoelectric (TE) and/or optoelectronic applications. This patent is limited to type II clathrates and similarly to those discussed above, does not disclose the production of phase pure high quality single crystals of both type I and type II clathrates.
As shown by the disclosures of the prior art, a method of high quality crystal growth of the type-I and type-II clathrates is needed. The present invention offers a new, industrially accessible and cost-effective method for crystal growth of intermetallic materials. The method of the invention enables the growth of high quality single crystals of materials including constituent elements with greatly differing melting points and vapor pressures, or both.